PREP: Line an 8x8-inch or 9x9-inch pan for thicker fudge (or 11x7-inch pan for thinner fudge) with parchment paper, leaving an overhang for easy removal (or line with foil) - do not skip this or you probably won't be able to get your fudge out looking very nice. (The fudge photographed was made in an 8x8-inch pan) Cut the butter into 1-tablespoon-sized pieces. Coarsely chop the graham crackers and add them to a large bowl. Measure out the marshmallows and add to the large bowl with the graham crackers.
COMBINE: In a thick-bottomed, nonstick pan, heat the butter, chocolate chips, and sweetened condensed milk using the lowest heat setting. Using a heat-resistant spatula, stir the mixture constantly, making sure to scrape the sides and bottom. Resist the urge to increase the heat-- your patience will pay off!
ADDFLAVOR: As soon as the butter and chocolate chips are melted, remove the pan from the heat, and stir in the salt and vanilla extract. Mix well. Pour the mixture into the bowl with the prepared graham crackers and marshmallows. Gently stir to combine; it's okay if the chocolate melts down the marshmallows a little.
CHILL: Transfer the mixture to the lined pan. Spray a spatula with nonstick cooking spray and smooth the chocolate fudge into one even layer that is touching all sides of the pan and has a smooth top. Cover and refrigerate for 4-8 hours or until fudge is firm.
SERVE: Remove from the pan using the overhang of parchment paper. Cut the fudge with a hot, very sharp knife (run it under hot water, dry, and then make quick cuts). I recommend cutting the fudge into small pieces -- it's rich!
STORAGE: This fudge is best stored in the fridge and enjoyed cold. If left out of the fridge for long, the fudge becomes crumbly. Leftover fudge will stay good for 1-2 weeks in an airtight container in the fridge, but because of the marshmallows, we like it best within 2-3 days.
Notes
Note 1: Milk chocolate is our favorite for S'mores Fudge, but it's quite rich. Use semi-sweet or dark chocolate for a more bitter (less sweet) tasting fudge, or use half semi-sweet and half milk chocolate. My favorite chips to use are Ghirardelli® or Guittard®. The higher the quality of chocolate, the tastier your fudge will be!